vfs: re-use the File objects when re-reading the directory

Make it so that d.items is never nil to simplify the code

This should help with inconsistent reads when the source object changes.

vfs/dir.go

@@ -24,7 +24,7 @@ type Dir struct {
 	entry   fs.Directory
 	mu      sync.Mutex      // protects the following
 	read    time.Time       // time directory entry last read
-	items   map[string]Node // NB can be nil when directory not read yet
+	items   map[string]Node // directory entries - can be empty but not nil
 }
 
 func newDir(vfs *VFS, f fs.Fs, parent *Dir, fsDir fs.Directory) *Dir {
@@ -36,6 +36,7 @@ func newDir(vfs *VFS, f fs.Fs, parent *Dir, fsDir fs.Directory) *Dir {
 		path:    fsDir.Remote(),
 		modTime: fsDir.ModTime(),
 		inode:   newInode(),
+		items:   make(map[string]Node),
 	}
 }
 
@@ -111,7 +112,7 @@ func (d *Dir) ForgetPath(relativePath string) {
 	d.walk(absPath, func(dir *Dir) {
 		fs.Debugf(dir.path, "forgetting directory cache")
 		dir.read = time.Time{}
-		dir.items = nil
+		dir.items = make(map[string]Node)
 	})
 }
 
@@ -122,11 +123,9 @@ func (d *Dir) ForgetPath(relativePath string) {
 func (d *Dir) walk(absPath string, fun func(*Dir)) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
-	if d.items != nil {
-		for _, node := range d.items {
-			if dir, ok := node.(*Dir); ok {
-				dir.walk(absPath, fun)
-			}
+	for _, node := range d.items {
+		if dir, ok := node.(*Dir); ok {
+			dir.walk(absPath, fun)
 		}
 	}
 
@@ -153,25 +152,21 @@ func (d *Dir) rename(newParent *Dir, fsDir fs.Directory) {
 // note that we add new objects rather than updating old ones
 func (d *Dir) addObject(node Node) {
 	d.mu.Lock()
-	if d.items != nil {
-		d.items[node.Name()] = node
-	}
+	d.items[node.Name()] = node
 	d.mu.Unlock()
 }
 
 // delObject removes an object from the directory
 func (d *Dir) delObject(leaf string) {
 	d.mu.Lock()
-	if d.items != nil {
-		delete(d.items, leaf)
-	}
+	delete(d.items, leaf)
 	d.mu.Unlock()
 }
 
 // read the directory and sets d.items - must be called with the lock held
 func (d *Dir) _readDir() error {
 	when := time.Now()
-	if d.read.IsZero() || d.items == nil {
+	if d.read.IsZero() {
 		// fs.Debugf(d.path, "Reading directory")
 	} else {
 		age := when.Sub(d.read)
@@ -187,39 +182,40 @@ func (d *Dir) _readDir() error {
 	} else if err != nil {
 		return err
 	}
-	// NB when we re-read a directory after its cache has expired
-	// we drop the old files which should lead to correct
-	// behaviour but may not be very efficient.
-
-	// Keep a note of the previous contents of the directory
-	oldItems := d.items
 
 	// Cache the items by name
-	d.items = make(map[string]Node, len(entries))
+	found := make(map[string]struct{})
 	for _, entry := range entries {
+		name := path.Base(entry.Remote())
+		node := d.items[name]
+		found[name] = struct{}{}
 		switch item := entry.(type) {
 		case fs.Object:
 			obj := item
-			name := path.Base(obj.Remote())
-			d.items[name] = newFile(d, obj, name)
+			// Reuse old file value if it exists
+			if file, ok := node.(*File); node != nil && ok {
+				file.setObjectNoUpdate(obj)
+			} else {
+				node = newFile(d, obj, name)
+			}
 		case fs.Directory:
 			dir := item
-			name := path.Base(dir.Remote())
-			// Use old dir value if it exists
-			if oldItems != nil {
-				if oldNode, ok := oldItems[name]; ok {
-					if oldNode.IsDir() {
-						d.items[name] = oldNode
-						continue
-					}
-				}
+			// Reuse old dir value if it exists
+			if node == nil || !node.IsDir() {
+				node = newDir(d.vfs, d.f, d, dir)
 			}
-			d.items[name] = newDir(d.vfs, d.f, d, dir)
 		default:
 			err = errors.Errorf("unknown type %T", item)
 			fs.Errorf(d, "readDir error: %v", err)
 			return err
 		}
+		d.items[name] = node
+	}
+	// delete unused entries
+	for name := range d.items {
+		if _, ok := found[name]; !ok {
+			delete(d.items, name)
+		}
 	}
 	d.read = when
 	return nil

vfs/file.go

@@ -252,6 +252,14 @@ func (f *File) setObject(o fs.Object) {
 	f.d.addObject(f)
 }
 
+// Update the object but don't update the directory cache - for use by
+// the directory cache
+func (f *File) setObjectNoUpdate(o fs.Object) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.o = o
+}
+
 // Get the current fs.Object - may be nil
 func (f *File) getObject() fs.Object {
 	f.mu.Lock()